This invention relates to fluorescent gaseous discharge lamps of the type commonly used as a source of artificial illumination and, more particularly, to a filtering system for improving the spectral output of such lamps.
Fluorescent lamps in common use today comprise a tubular envelope with electron-emitting electrodes at each end. The envelope is filled with a low pressure ionizable medium such as mercury vapor and a rare gas such as argon. When a suitable voltage is applied to the electrodes, an electrical discharge takes place in the mercury vapor which produces resonance radiation of the ionized mercury. This radiation occurs at a number of different wavelengths characteristic of mercury often referred to as the mercury lines. Part of this radiation is in the ultraviolet range and part is in the visible range. The ultraviolet part excites a phosphor coating on the inside of the lamp envelope which fluoresces to produce visible light emitted by the lamp. The lamp envelope is designed to attenuate a substantial portion of the ultraviolet emitted by the mercury arc which may be harmful and to pass the visible light from the irradiated phosphor coating. However, the lamp envelope also passes the mercury art radiation in the visible range which causes distortion in the spectral light energy distribution curve of the lamp.
It is believed that the spectral distortions in the light output of fluorescent lamps caused by mercury radiation are a form of light pollution which may produce undesirable biological effects in plants, animals and man. Research done in this field indicates that natural light from the sun is a very important factor affecting life on earth and that artificial light sources should approach natural daylight in spectral distribution as closely as possible. For that reason it is desirable to eliminate from the light output of fluorescent lamps the spectral distortions caused by mercury radiation so that the lamp produces light with a smooth spectral energy distribution curve more nearly resembling natural daylight.
Accordingly, it is an object of this invention to provide an improved fluorescent lamp producing a light output which is free of the spectral distortions caused by arc radiation.
A further object of the invention is to provide a filtering system for a fluorescent lamp which prevents mercury arc radiation in the visible range from passing outside of the lamp without interfering with the ultraviolet radiation which excites the phosphor coating on the lamp envelope to produce the desired light output of the lamp.
Further objects and advantages of the invention will become apparent as the following description proceeds.